Quad TVS protection circuit for an electronic DSL component

ABSTRACT

A protection circuit for use with an electronic DSL component having a tip connection and a ring connection, the protection circuit including: a first unidirectional transient-voltage-suppression (TVS) diode, having a negative TVS breakdown voltage BDV and diode forward voltage DV clamp, connected between Vcc and the tip connection of the DSL component; a second unidirectional TVS diode, having a diode forward voltage DV, connected between the tip connection of the DSL component and a negative ground clamp node; a third unidirectional TVS diode, having a negative TVS breakdown voltage BDV and diode forward voltage DV clamp, connected between Vcc and the ring connection of the DSL component; and a fourth unidirectional TVS diode, having a diode forward voltage DV, connected between the ring connection of the DSL component and the negative ground clamp node.

BACKGROUND

Digital Subscriber Line (DSL) refers to technologies for communicationof data over telephone lines. DSL technology is built into variouselectronic devices for providing various types of data communication,such as internet connection, VOIP, and audio/video streaming services,for a premises. Fundamentally, DSL technologies can be implemented in amodem, but depending on the functionalities provided, the DSL device canbe referred to as a router, gateway, or access point.

It has been found in some DSL devices, (e.g., the BGW210 gateway deviceincluding a BCM6303 line driver IC) that under certain conditions alatent DSL line driver failure may occur. For example, it has beenobserved that after an electrostatic discharge (ESD) contact dischargetest, the DSL interface of the device no longer worked. For example,testing of devices for electrostatic discharge (ESD) contact dischargeup to +/−8 kV showed device failures. Further, it has been found thatESD contact discharge greater than +/−4 kV can compromise the DSL linedriver IC, which can present as electrical over-stress (EOS) damage.Such compromised devices could then prematurely fail under subsequentpower reboot cycles. ESD testing followed by repetitive power cyclespromoted premature DSL line driver failure. Premature failures werefound to be the result of progressive ESD damage (EOS) accelerated byrepetitive power cycles. Observations included: a) The line driver casetemperature would rise above nominal 5 or more degrees Celsius upon ESDEOS damage; b) EOS damaged parts would typically operate yetprogressively worsen until failure upon multiple power cycles; and c)Most EOS tests failed in the “negative” ESD pulse direction.

Root cause analysis identified damaged ESD transistors in the ESDrail-clamp between two internal power rails. Furthermore, the ESDingress conduction path was identified to be the differential outputpin(s) to the power and/or ground.

The present disclosure sets forth the ESD circuit failure to beaddressed, compares related protection circuit elements and metrics,provides a list of key technical specifications, and provides newcomponents that eliminate the failure. Due to the complex ESD testconfiguration with high voltage and fast signal switching speeds, theresulting Electro Magnetic Pulse (EMP) tends to prevent directmeasurement of voltages and currents required for direct failureanalysis. Therefore, a series of experiments was used to provide theparameters for the new circuits disclosed herein which represent asolution to the previous component failures.

SUMMARY

A first aspect of the disclosure is a protection circuit for use with anelectronic DSL component having a tip connection and a ring connection,the protection circuit including a first unidirectionaltransient-voltage-suppression (TVS) diode, having a negative breakdownvoltage BDV and a diode forward voltage DV connected between Vcc and thetip connection of the DSL component; a second unidirectional TVS diode,having a negative breakdown voltage BDV and a diode forward voltage DVconnected between the tip connection of the DSL component and a negativeground clamp node; a third unidirectional TVS diode, having a negativebreakdown voltage BDV and a diode forward voltage DV connected betweenVcc and the ring connection of the DSL component; and a fourthunidirectional TVS diode, having a negative breakdown voltage BDV and adiode forward voltage DV connected between the ring connection of theDSL component and the negative ground clamp node.

A second aspect is that a TVS clamp amount for a negative common modevoltage on the tip and ring connections is substantially Vcc minus BDVand positive common mode voltage on the tip and ring connections issubstantially Vcc plus DV.

A third aspect is that a diode clamp amount for a secondary negativecommon mode voltage clamp on the tip and ring connections issubstantially negative ground clamp minus DV.

A fourth aspect is that a clamp amount of a differential mode voltagebetween the tip and ring connections is substantially BDV.

FIG. 1 shows the DSL line driver, BCM6303, Output Stage Block Diagram.

FIGS. 2A, 2B, and 2C show the ESD clamp schematic.

FIG. 3 shows the tested DSL secondary protection circuit.

FIG. 4 shows the proposed PCB change.

FIGS. 5A-5F show examples of tested circuits.

FIG. 6A-6J show comparative circuit examples for ESD secondary protectorclamping path.

FIG. 7A through 7D show BGW210 ESD improved design rate vs reachbaseline comparison.

FIG. 8 shows ESD results.

FIG. 9 shows absolute maximum power ratings.

FIG. 10 shows electrical characteristics at 25 degrees Celsius.

DETAILED DESCRIPTION

Evaluation of devices that had failed testing, and which had thereforeexhibited DSL interface failures, revealed ESD rail clamp transistordamage and ESD ingress through the line driver output pin of the linedriver IC. Both differential (+/−) pins were found to be subject to ESDdamage and failure. Disclosed herein are exemplary protection circuitsincluded in a DSL device (e.g., modem, gateway) that achieve ESD contactprotection greater than +/−4 kV on the DSL line driver IC included inthe DSP device.

The ESD test configuration is complex (ESD Gun; Device Under Test (DUT);AC/DC Power Supply; Test Ground; ESD Gun Ground Return) with highvoltage fast switching signals (<1 nS) resulting in large ElectroMagnetic Pulse (EMP) disturbances preventing direct measurement ofcurrents and voltages for analysis. According to an example contactdischarge specification (e.g., DIRECTV's DTV CPDV-HPL-0099 ESD ContactDischarge specification), the test discharge was done individually oneach DSL line driver TIP/RING pin. The discharge was made through sixfeet of CAT-3 telephone wire. The received ESD energy at the DSL modemRJ14 registered jack (RJ) is complex, primarily common mode yet with asmall differential mode component resulting from the telephone wiretwisted pair inter-twist capacitive coupling. The received ESD pulse istransferred to the DSL line driver secondary protection circuit throughthe DSL transformer inter-winding capacitance with similar ESD pulsepolarity and voltage modes.

A series of experiments was used to identify an ESD solution thatimproves the DSL line driver ESD immunity. Design improvements wereobtained from a series of ESD contact discharge tests. Many tests wereperformed to limit or reduce the ESD exposure to the line driver outputpin(s), or to limit the power supply conduction current. Various currentlimiting devices, (e.g., TBU (Bourns Transient Blocking Unit) and TCS(Bourns Transient Current Suppressor)), were tried in the power feed andline driver output pin signal path. Various combinations ofbi-directional transient-voltage suppression (TVS) clamps were tried toreduce common mode voltage at the line driver output pins, and to limitthe +12V power excursions. Secondary Surge/ESD protector device parallelpower/ground clamping diodes were tried to improve common mode clampingof voltages reaching the line driver. Some experiments improved ESDimmunity better than others.

With the knowledge that common mode clamping was key, use ofunidirectional TVS diodes, (e.g., Littelfuse SP4022), were tested inparallel with the existing Protek TVS device steering diodes. Acceptableperformance was achieved on five modified units. Further design reviewled to the determination that the resulting circuit was redundant withthe existing Protek part. The Protek part was removed just using theunidirectional TVS diodes in its place.

As explained above, ESD ingress was determined to be the line driveroutput pin(s) with egress through the power and ground pins. The ESDdamaged portion of the line driver die was determined to be an ESD RailClamp circuit internal in the DSL line driver IC. The Rail Clamp ESDconduction path is illustrated in FIG. 1 , (showing the test example DSLline driver, BCM6303, Output Stage Block Diagram). FIG. 1 also showsfailure of the ESD rail clamp.

FIGS. 2A and 2B show the ESD clamp schematic. The exemplary new circuitdisclosed herein and shown in FIG. 2C replaces the circuit of FIG. 2Band avoids ESD damage by improving the secondary protector common modeclamp on the DSL transmission path.

Consideration was given to using a 3.3V to 12V boost regulator asopposed to a direct power connection to avoid ESD damage due to thepossibility that a direct connection to the power supply is capable oflarger damaging currents delivered to the line driver when the ESD railclamps are activated in the fault condition. Such a boost regulator isnot as “stiff” as the direct power connection, and as a result may not“fold-over” during the line driver ESD fault condition eliminatingdamaging currents in the line driver ESD rail clamp.

In FIG. 1 , the ESD conduction paths labeled 11 represent current flowfrom the +12V power supply to an ESD negative going signal on the outputpin. This negative ESD conduction path represents the majority of ESDtest failures in the negative pulse direction. With most of the failuresin a negative ESD pulse test, negative common mode mitigation isdesirable for improved ESD immunity of the line driver.

BGW210 HW2.5 Line Driver Power Supply Current Limiting

With the evidence indicating that the +12V power supply stiffness wasrelated to ESD immunity and rail clamp damage, both TBU (BournsTransient Blocking Unit) and TCS (Bourns Transient Current Suppressor)current limiting devices were tested for ESD immunity improvement.Different current limit levels (250 ma/500 mA) and topologies (combinedline drivers and individually) were tried to mitigate ESD rail clampdamage (by limiting +12V current) during the ESD event. These tests wereinconclusive, and did not provide adequate margin to be considered aviable ESD mitigation solution.

The conclusion was that there is sufficient energy in the 132 uF +12VMLCC decoupling caps to damage the ESD rail clamp transistors. Simplysoftening the power supply was insufficient to guarantee 8 kV ESDcontact discharge performance of the BGW210 HW2.5 design.

BCM6303 Output Pin TVS Clamping

Adding 12V bi-directional common and differential mode TVS diodes (threebi-directional TVS diodes, two from TIP/RING to ground and onedifferentially between TIP and RING) at the line driver output pins didnot meet the 8 kV ESD contact discharge requirement. This providedadditional evidence that DSL transformer secondary protectionimprovement was required.

BCM6303 Line Driver Output Current Interrupter

Knowing the ESD discharge path was through the line driver outputpin(s), use of TBU/TCS current limiting/interrupting devices was triedto eliminate damaging current within the line driver device. These testswere inconclusive by not providing adequate margin to be considered aviable ESD mitigation solution.

SCR Secondary Protection Clamping Device

A silicon controlled rectifier type device, (e.g., Littelfuse DSLP024)was tested in place of the Protek TVS device with added 1M Ohm resistorand +12V clamping diode on the +bias pin. One tested unit passed 8 kV,and failed at 9 kV. This approach has no negative common mode clampimprovement, so was not fully explored.

Improved Secondary Protection Common Mode Clamps

Understanding reduced ESD signal swing at the line driver output pinswas a mitigation goal, improved secondary common mode clamping diodeswere incorporated. Use of the Littelfuse SP4022 (12V) unidirectionalpart was selected because of the fast P/N steering diode to be used inparallel with the Protek internal steering diodes.

FIGS. 2-4 , ESD negative Common Mode (−CM)/Littelfuse only clamp circuitpassing 9 kV, is from test results of FIG. 5 used to determine thecircuit design disclosed herein.

FIG. 5A shows the Littelfuse SP4022 device in parallel with the ProtekPDT5178 steering diodes. This configuration passed up to 9 kV ESDcontact discharge level, exceeding the 8 kV requirement.

The circuit in FIG. 5B eliminated the lower +CM ground clamp diodes ofthe SP4022 device. This configuration also passed up to +/−9 kV ESDcontact discharge level. This is where the ESD mitigation −CM (line 51in FIG. 5F, lines 61 in FIG. 6J) improvement discovery was made,confirmed by the FIG. 5 following two tests leading to the proposeddesign improvement disclosed herein. The upper unidirectional TVS diodein the SP4022 provides the −CM improved margin.

The configuration in FIG. 5C shows only the lower parallel +CM SP4022parts under test. No −CM ESD contact discharge improvement was noted.This confirmed the observation for the upper TVS diodes improvement justnoted above.

And finally, in the fourth configuration, shown in FIG. 5D, removingredundant Protek PDT5178 protection part and testing only the LittelfuseSP4022 TVS parts was investigated. This confirmed the 9 kV ESD contactdischarge level improved design. Accordingly, the proposed ESDconfiguration has been reached.

As shown in FIGS. 6A-6J, BGW210 HW2.5 ESD Secondary Protector ClampingPath Comparison, it is noted that the −CM clamping voltage of the Protekdevices is approximately −26V while the design disclosed herein providesa −CM clamping voltage of approximately −4V. Therefore, the new designdisclosed herein does not allow such a large voltage swing. Theconclusion was the Littelfuse SP4022 TVS diodes work better than theProtek TVS/steering diodes, but more importantly the SP4022 provided animproved −CM negative signal clamp for the +12V power rail. The latterappears to be a basic design change that provides increased −CM ESDimmunity. The SP4022 improved ESD common mode clamp conduction path isshown by the green traces 61 in FIG. 6J. This is proposed as the keychange improving ESD immunity for the BGW210 HW2.5 design.

This discovery led to the proposed solution for the BGW210 HW2.5 ESDimmunity deficiency. As part of the decision, further consideration ofthe impact on DSL swing led to the adoption and test of the LittelfuseSP4023, a 15V part, which provides slightly more headroom for the DSLsignal. The nominal differential and common mode clamping levels weretested as shown in FIGS. 3 and 4 and take into consideration the finalproposal using the Littelfuse SP4023 unidirectional TVS device.

Please note, careful review of FIGS. 2A-2C, BGW210 HW2.5 ESD SecondaryProtector Clamping Path Comparison, shows design compliance with thesignaling levels at the DSL transformer secondary protection circuit.The DSL signal swing at the secondary protection circuit and transformeris differentially 11 Vpp, or −0.25V to 5.25V single ended. Forreference, the signaling at the line driver outputs are differentially˜22 Vpp, or −2.95V to +7.95V single ended.

Proposed ESD Circuit Improvement

FIG. 2B shows the old (Protek PDT5178) secondary protection circuit andFIG. 2C shows the new (Littelfuse SP4023) secondary protection circuit.This proposal is to replace the Protek device with four Littelfuse TVS,or similar, parts on the top side of the circuit board improving −CM ESDsurge protection. The plan is to provide a Protek back-up stuff optionon the bottom side of the board, providing PCB space allows such aconfiguration. The circuit of FIG. 2C connects to the tip line 20 andring line 22 going to the protected electronic DSL component 200 (a partof which is shown in the upper left corner of FIG. 2A).

Testing has shown the proposed Littelfuse design change improves ESDimmunity, while maintaining both Surge and DSL performance levels. ForDSL RvR results, please refer to FIGS. 7A-7D, BGW210 ESD Improved DesignRate vs Reach Baseline Comparison.

To summarize FIG. 6A-6J, inference from measured results show thischange provides improved negative common mode voltage clamping at theline driver output pin. By clamping with respect to the +12V power railand minimizing the negative swing, the internal ESD rail clamp is underless stress. The updated design also provides earlier activation of thedifferential clamping between the TIP and RING lines.

FIG. 3 shows the tested DSL secondary protection circuit. The Protekdevice was removed, and the Littelfuse parts installed (stacked) acrossthe pads for the high-side common mode clamp, and piggybacked on top forthe low-side. FIG. 4 shows a mock-up of the proposed PCB change. Theplan is for the Protek device to be a “noinstall” option on the bottomside of the board.

DSL Secondary Protector Modified Design—ESD/Surge/DSL RvR Test Results

Ten BGW210 HW2.5 boards were tested with the proposed Littelfuse SP4023(15V) replacement of the Protect PDT5178 part. All ten modified boardspassed both the 8kV ESD contact discharge requirement and the 200-bootcycle test. No trouble was found on any of the modified boards. Inaddition, two boards (SN R91NG8HL100054, R91NG8HL100246) passed the DSLSurge test requirement, and two other ESD stressed boards (SNR91NG8HL100XXX, R91NG8HL100267) passed baseline DSL Rate versus Reach(RvR) tests (please see FIGS. 7A-7D).

Some of the boards were tested to failure after passing 9 kV initiallyfollowed by the 200 reboot test. The boards were tested to failure wherea DSL line driver case temperature rise was noted. From the availableresults, all units passed at a level of 8 kV (or higher)+200 rebootcycles. Of the eight units tested to failure (TEMP rise), one 9 kV, one10 kV, and six 14 kV ESD failures were observed. All ESD stressed unitspassed the 8 kV contact discharge requirement.

ESD/Surge Protection Device Electrical Specifications

The Protek PDT5178 is the TVS w/steering diodes (two signal) partcurrently in production use on the BGW210. The Littelfuse SP4023 and thealternate Protek GBLC15 are the TVS w/steering diodes (single signal)parts proposed for use on BGW210 in this document. Though the parts arenot an exact match, and offer slightly different parametricspecifications, they are comparable in ESD and Surge performance levels.Furthermore, the worst case capacitive loading is currently the best forthe Littelfuse solution, resulting in similar DSL performance comparedwith the Protek part.

FIGS. 7A through 7D show BGW210 ESD improved design rate vs reachbaseline comparison.

FIG. 8 shows ESD results.

FIG. 9 shows absolute maximum power ratings.

FIG. 10 shows electrical characteristics at 25 degrees Celsius.

CONCLUSIONS

Though direct measurement of ESD induced signals are impractical,inference from targeted statistical test results provided a BGW210 HW2.5ESD contact discharge immunity improvement solution. Along with improvedESD performance, the proposed design change has similar performance ofDSL Surge and RvR results. The proposed change is an improvement to theexisting production BGW210 HW2.5 design using similar ESD and surgeprotection TVS/Steering diode methodologies for signal/power clampprotection with improved (lower) ESD-CM clamp.

The proposed change is confined to a small area of the overall BGW210board, not requiring retesting of other sub-systems and circuits in thedesign. The proposed change has limited impact on the PCB floorplan. Thechanges are planned to maintain the existing Protek protection device asa back-up stuff option on the bottom side of the board.

The proposed change improves both differential and common modeprotection against ESD and Surge damaging events. There is little riskin making this change, and measured statistical gains have been notedfor the design change of replacing the DSL secondary protector device(Protek PDT5178) with four unidirectional TVS devices (Littelfuse SP4023or equivalent).

The proposed DSL secondary protector change allows the BGW210 HW2.5 tosatisfy the required 8 kV ESD contact discharge specification withoutcompromise of the overall product release.

The invention claimed is:
 1. A protection circuit for use with anelectronic DSL component having a tip connection and a ring connection,said protection circuit comprising: a first unidirectionaltransient-voltage-suppression (TVS) diode array, having a negativebreakdown voltage BDV and a diode forward voltage DV connected between asupply voltage (Vcc) and the tip connection of the DSL component; asecond unidirectional TVS diode array, having a negative breakdownvoltage BDV and a diode forward voltage DV connected between the tipconnection of the DSL component and a negative ground clamp node; athird unidirectional TVS diode array, having a negative breakdownvoltage BDV and a diode forward voltage DV connected between Vcc and thering connection of the DSL component; and a fourth unidirectional TVSdiode array, having a negative breakdown voltage BDV and a diode forwardvoltage DV connected between the ring connection of the DSL componentand the negative ground clamp node; wherein the tip connection to theVcc via the first unidirectional TVS diode array and the ring connectionto the Vcc via the third unidirectional TVS diode array forms anelectrostatic common mode clamp conduction path so as to provide anegative common mode (CM) negative signal clamp for Vcc.
 2. Theprotection circuit according to claim 1, wherein a TVS clamp amount fora negative common mode voltage on the tip and ring connections issubstantially Vcc minus BDV and positive common mode voltage on the tipand ring connections is substantially Vcc plus DV.
 3. The protectioncircuit according to claim 1, wherein a diode clamp amount for asecondary negative common mode voltage clamp on the tip and ringconnections is substantially voltage at negative ground clamp minus DV.4. The protection circuit according to claim 1, wherein a clamp amountof a differential mode voltage between the tip and ring connections issubstantially BDV.